Rinse quinoa under cold running water and drain. Combine water and quinoa in a medium saucepan and bring to a boil. Reduce heat to a simmer, cover and cook until quinoa is tender and all the water is absorbed, 15 to 20 minutes. Place quinoa in a large bowl with watercress, pears, pomegranate seeds, walnuts, juice and vinegar. Stir to combine, then serve. Serves 4.